When some one left without signing out from his/her mail box , you can receive his /her mails in your mail box & send emails to his/her friends/others as if he/ she sent that email for life time.
yaa that's true. by using the services provided by gmail u can do this....

process 1: how to receive his emails afterwardsif he/she uses gmail:
go to setting >> forwarding & POP >> check forwarding option >> enter your email id>> save settings, thats all . you will receive emails in your in box that are sent to his/her idif he/she use yahoo mail:
go to options [right up corner]>> check forwarding box >> enter your email id click save>> go to your in box >> check mail>> click on confirmation email. thats all.you will receive emails in your in box that are sent to his/her idprocess2: how to send emails by his/her email address
go to your gmail>> go to settings>> click on the accounts tab>> click add another email >> enter his email id>> go to his/her inbox >> check in box >> click confirmation link
now you can send emails using his/her id from your email , go to compose mail toggle between your id and his her id i.e. you can send emails as his/her id